The CATIE Alzheimer's Disease Trial is part of the Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE) Project. The study is for people with Alzheimer's disease who are having trouble with their thinking or behavior. In particular, this study is trying to find out the best treatment for people who have hallucinations (seeing or hearing things that aren't there), delusions (false beliefs), or agitation. The design of the trial helps to increase the chance that participants in the study receive a medication that helps them. The study uses three medications known as atypical antipsychotics (olanzapine, quetiapine, risperidone), which are the newest medications that are currently available for treating these problems. Participants may also receive an antidepressant (citalopram). The trial lasts for 36 weeks. Participants are given a thorough evaluation at no cost to ensure that this study is appropriate. In addition, the caregiver, family member, or friend who comes with the participant will be offered an educational program about Alzheimer's disease.